1. Field of the Invention
The present invention relates to an intermediate casing for a turbofan engine, in particular for an aircraft turbofan.
2. Description of the Related Art
A twin-spool jet engine comprises conventionally, from upstream to downstream in the direction of flow of air or gas, a low-pressure compressor, a high-pressure compressor, a combustion chamber, a high-pressure turbine and a low-pressure turbine, which define a primary gas flow. The high-pressure turbine is constrained to rotate with the high-pressure compressor, via a so-called high-pressure shaft, so as to form a high-pressure body, while the low-pressure turbine is constrained to rotate with the low-pressure compressor via a low-pressure shaft so as to form a low-pressure body. Each turbine thus drives the associated compressor in rotation about an axis of the turbofan, under the effect of the thrust of the gases issuing from the combustion chamber.
Moreover, a twin-spool turbofan comprises a fan, surrounded or streamlined by a nacelle and able to generate a flow of air referred to as the secondary flow, said fan generally being rotated by the low-pressure turbine.
In a double-flow twin-spool turbofan, the term intermediate casing is given to a casing comprising a radially inner hub and a radially outer barrel, connected by radial arms, part of the annular separation being situated radially between the inner hub and the outer barrel. The inner hub, the outer barrel and the separation part are coaxial, the separation part having the radial arms passing through it. These arms are structural, that is to say are arranged so as to support elements of the structure and absorb and/or transmit forces.
The inner hub and the separation part delimit a part of the flow stream of the primary flow, referred to as the primary stream. The separation part and the outer barrel delimit a part of the flow stream of the secondary flow, referred to as the secondary stream. The radial arms pass through the primary and secondary streams.
At least one of the aforementioned arms is hollow and allows services to pass. These services may be:                pipes through which hydraulic fluids pass, such as pipes supplying or returning lubricating or cooling oil,        mechanical members transmitting power, for driving in particular the accessory gearbox (AGB),        ventilation channels,        electric cables, connected for example to various sensors.        
The patent FR 2 921 973 discloses an intermediate casing for a turbofan comprising an inner hub and an outer barrel connected by radial arms, at least one of which is hollow and houses a transmission shaft arranged to take off the power from the shafts of the high-pressure and low-pressure bodies in order to transfer it to the accessory gearbox with a view to driving auxiliary devices. The hollow arm passes through an annular separation part.
The internal space of the hollow arm makes it possible to discharge oil coming from the inner hub and serving to lubricate bearings supporting the aforementioned shafts of the low-pressure and high-pressure bodies.
The main cross section of these hollow arms is relatively high. In other words, the cross section of flow enabling oil to pass inside the hollow arm, round the transmission shaft, is relatively great. The main cross section is the largest dimension of the arm perpendicular to the chord.
There currently exists a need to reduce the influence of the structural arms on the flow of the primary and secondary flows. One solution is then to reduce the main cross section of the arms, in particular of the hollow arm, which limits the cross section of flow of oil, in particular the cross section of flow of oil around the transmission shaft.
There also exists a need to guarantee the circulation of oil from a region situated upstream of said transmission shaft to an outlet connected to the filtering and pumping equipment, and to equip the arm with an oil outlet situated downstream of the transmission shaft, while the oil to be discharged may come from a region situated upstream of said transmission shaft. The flow of oil that can be brought from the upstream region as far as the downstream outlet is then limited by the cross section of flow between the walls of the hollow arm and the transmission shaft. This cross section and the corresponding oil flow are more limited, the smaller the main cross section.
The terms upstream and downstream are defined here with respect to the axis of the turbofan and with reference to the direction of circulation of the primary and secondary flows.